1. Field of the Invention
The present invention relates to a monitoring system to monitor a network system in which a plurality of transmitting apparatuses are connected to a communication medium and information is transmitted.
2. Related Background Art
In recent years, various kinds of local area network systems (hereinafter, referred to as LAN) in which a plurality of business machines such as work stations, office computers, personal computers, etc. are connected by a common network transmission path have been put into practical use.
Hitherto, a special network monitoring system has been used as a tool to check the operation and to analyze the fault in the development of various kinds of connecting apparatuses for use in LAN and in the operation of the system.
FIG. 6 shows a conventional example of the connection between the LAN and the network monitoring apparatus. Generally, in the LAN, many token passing methods whereby only the apparatus which received the token as a well-known communication right transfer instruction can acquire the transmission right are used.
In FIG. 6, reference mumeral 1 denotes a network transmission path; 60a to 60d indicate transmitting apparatuses (hereinafter, referred to as nodes); and 61 represents a network monitor.
The network monitor 61 operates as one node on the network and is a special apparatus which is used for the monitor test to receive all of the data on the network, simulation test to communicate a message to the other arbitrary node through the network, and the like.
Therefore, a special monitoring apparatus must be purchased and installed, causing the overall costs to be raised and the installing space to be used wastefully.
To solve these disadvantages, there has also been developed a system having a structure such that the communication media monitoring function is also provided for the general node in place of the special monitoring apparatus.
However, the foregoing conventional systems have the following drawbacks.
There is an inconvenience such that when the operating mode of the node connected to the LAN is changed from the normal operating mode to the media monitoring operating mode, the token is temporarily lost.
In other words, according to the token passing method, the control information (communication right transfer instruction) indicative of the communication right acquisition permission called a token is circulated in the network and only the node which acquired the token or only the node designated by the token can obtain the transmission right. However, after the node is set into the media monitoring operating mode, it never performs the transmitting operation. Therefore, even if the token is given to this node, it does not transfer the token to the other node.
Consequently, when this node is seen from the other nodes, it is regarded as if the power source was disconnected. Therefore, the token lost recovery process is executed by the other node. During this recovery interval, the communication is stopped for a long time. On the other hand, complicated processing procedures are necessary even in the case of performing the recovery process